1. Field of the Invention
The present invention relates generally to piezoelectric optical switches, and particularly to planar Mach-Zehnder piezoelectric optical switches having low birefringence and a high extinction ratio.
2. Technical Background
As the demand for bandwidth increases, so does the drive toward intelligent, low-cost, and dynamically-reconfigurable fiber optic networks. To bring this to pass, network designers are seeking ways to replace certain network functions that were traditionally performed in the electrical domain with solutions in the optical domain, as economics and system designs permit. Designers have recognized for quite some time that four port optical devices could find widespread application in fiber networks to provide fault tolerance, signal modulation, and signal routing. Integrated optical devices using either thermo-optical or electro-optical techniques are currently available. However, these devices have drawbacks due to high power consumption and low switching speeds.
Four-port piezoelectric optical devices are of particular interest because of their lower power consumption, reduced switching time and adaptability to mass production techniques, such as photolithography. One approach that has been considered involves an optical phase modulator fabricated by coating a fiber with a thick coaxial piezoelectric lead zirconate titanate film. This circular symmetric in-line fiber phase modulator provides phase modulation in a frequency range from 100 kHz to 25 MHz. Unfortunately, the efficiency of the device was poor as it exhibited high attenuation and low piezoelectricity because of difficulty of depositing a thick PZT film around an optical fiber.
In another approach that has been considered, a Mach-Zehnder fabricated from optical fibers was used to construct an optical switch. In this design, each optical fiber leg was positioned directly on a piezoelectric strip. This design also has several drawbacks. First, the piezoelectric strip required high voltage for commutation. Second, the positioning of the strip in relation to the fiber created asymmetrical stresses along the fiber axis that perturbed the polarization in the interferometer arms resulting in high birefringence and degraded cross-talk performance. As a consequence, polarized light is required when using this switch.
In yet another approach, a modulator was fabricated by laminating a piezoelectric strip on a planar waveguide device. The piezoelectric strip was formed by sandwiching a layer of piezoelectric material between a lower electrode and an upper electrode. The piezoelectric strip was then attached to the overclad of the device directly above the waveguide. However, when the piezoelectric strip was actuated, the resulting strain vector generated a strong birefringence effect that severely degraded the extinction ratio at the output of the device.
Thus, a need exists for a four port piezoelectric optical device having reduced birefringence characteristics, a high extinction ratio, lower power consumption, and reduced switching time. This switch must be cost effective, and its design suitable for mass production techniques.
The present invention is a four port piezoelectric optical switch that substantially solves the birefringence problem and addresses the other issues discussed above. In doing so, the piezoelectric switch of the present invention provides a high extinction ratio in addition to the lower power consumption and reduced switching time possible with piezoelectric switch technology. The planar design of the present invention is well suited for mass production techniques such as photolithography, and offers a promising low-cost solution for some of the signal routing and fault tolerance functionality needed to implement an intelligent fiber optic network.
One aspect of the present invention is an optical device for selectively directing a light signal in a direction of propagation, the optical device includes a propagation path for the light signal and an output. The optical device includes a piezoelectric element for directing the light signal into the output by creating a plurality of mutually orthogonal strain components in the optical device, wherein the piezoelectric element is disposed relative to the propagation path such that only a component of the plurality of mutually orthogonal strain components, aligned in the direction of propagation, may substantially exist in the propagation path.
In another aspect, the present invention is a Mach-Zehnder optical device for directing a light signal having a wavelength xcex in a direction of propagation. The optical device includes: a first waveguide having a first propagation path, a refractive index n, a first length L1, and a first output, wherein the light signal is propagated along the first propagation path; and a first piezoelectric rib for directing the light signal by creating a first plurality of mutually orthogonal strain components in the first waveguide, wherein the first piezoelectric rib is disposed on the first waveguide at a first offset from the first propagation path such that only a component of the first plurality of mutually orthogonal strain components that is aligned in the direction of propagation substantially may exist in the first propagation path.
In another aspect, the present invention is a method for directing a light signal having a wavelength xcex, in a direction of propagation in an optical device including a first waveguide having a first propagation path, a refractive index n, a first length L1, and a first output, wherein the light signal is propagated along the first propagation path. The method for directing a light signal includes the steps of: providing a first piezoelectric rib for generating a first plurality of mutually orthogonal strain components in the first waveguide, wherein the first piezoelectric rib is disposed on the first waveguide at a first offset from the first propagation path such that only a component of the first plurality of mutually orthogonal strain components that is aligned in the direction of propagation can substantially exist in the first propagation path; providing a second waveguide disposed adjacent to the first waveguide having a second propagation path, the refractive index n, a second length L2, and a second output; and actuating the first piezoelectric rib to selectively deform the first waveguide, wherein a first waveguide deformation produces the first plurality of mutually orthogonal strain components in the first waveguide.
In another aspect, the present invention discloses a method of fabricating an optical device used for directing a light signal. The method of fabricating includes the steps of: forming a substrate; disposing a waveguide core layer on the substrate; forming a first waveguide from the waveguide core layer, wherein the first waveguide structure is characterized by a first propagation path, a refractive index n, a first length L1, and a first axis, wherein the first axis is substantially perpendicular to the first length and the first propagation path; forming a second waveguide structure from the waveguide core layer, wherein the second waveguide structure is characterized by second propagation path, the refractive index n, a second length L1, and a second axis parallel to the first axis; disposing a first piezoelectric rib on the first waveguide structure, wherein the first piezoelectric rib has a first rib axis which is substantially parallel to the first axis and separated from the first axis by an offset; and disposing a second piezoelectric rib on the second waveguide structure, wherein the second piezoelectric rib has a second rib axis which is substantially parallel to the second axis and separated from the second axis by the offset, wherein the offset is selected to minimize a birefringence in the optical device.
In another aspect, the present invention discloses a method for selectively directing a light signal into a first output or a second output of an optical device that includes at least one waveguide having at least one core connected to the first output, wherein the light signal propagates along the at least one waveguide in a direction of propagation, the method for selectively directing a light signal comprising the steps of: providing at least one piezoelectric element for switching the light signal from the first output into the second output by inducing a plurality of mutually orthogonal strain components in the at least one waveguide, the at least one piezoelectric element being disposed on the at least one waveguide in a predetermined position such that only a first component of the plurality of mutually orthogonal strain components substantially exists in the at least one core, wherein the first component is a strain component aligned to the direction of propagation; and actuating the at least one piezoelectric element to thereby generate a deformation in the at least one waveguide causing the plurality of mutually orthogonal strain components to be produced in the at least one waveguide.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.